System and Method of Lattice-Based Search for Spoken Utterance Retrieval

ABSTRACT

A system and method are disclosed for retrieving audio segments from a spoken document. The spoken document preferably is one having moderate word error rates such as telephone calls or teleconferences. The method comprises converting speech associated with a spoken document into a lattice representation and indexing the lattice representation of speech. These steps are performed typically off-line. Upon receiving a query from a user, the method further comprises searching the indexed lattice representation of speech and returning retrieved audio segments from the spoken document that match the user query.

PRIORITY INFORMATION

This application is a continuation of U.S. patent application Ser. No.15/055,975, filed Feb. 29, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/200,700, filed Mar. 7, 2014, now U.S. Pat. No.9,286,980, issued Mar. 15, 2016, which is a continuation of U.S. patentapplication Ser. No. 13/052,819, filed Mar. 21, 2011, now U.S. Pat. No.8,670,977, filed Mar. 11, 2014, which is a continuation application ofU.S. patent application Ser. No. 10/923,915, filed on Aug. 23, 2004, nowU.S. Pat. No. 7,912,699, issued on Mar. 22, 2011, the content of whichare included herewith in their entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to retrieval of spoken documents and morespecifically to a system and method of performing a lattice-based searchfor retrieval of a spoken utterance.

2. Introduction

Automatic systems for indexing, archiving, searching and browsingthrough large amounts of spoken communications have become a reality inthe last decade. Most such systems use an automatic speech recognition(ASR) component to convert speech to text which is then used as an inputto a standard text based information retrieval (IR) component. Thisstrategy works reasonably well when speech recognition output is mostlycorrect or the documents are long enough so that some occurrences of thequery terms are recognized correctly. Most of the research in this areahas concentrated on retrieval of Broadcast News type of spoken documentswhere speech is relatively clean and the documents are relatively long.In addition, it is possible to find large amounts of text with similarcontent in order to build better language models and enhance retrievalthrough use of similar documents.

However, for contexts where spoken document retrieval is desirable butthe benefits of clean speech are unavailable, information retrievalbecomes more difficult. For example, if one were to record ateleconference and then desire to perform a search or informationretrieval of the portions of the conference, the problem becomes moredifficult. This is due to the fact that the teleconference likelyconsists of a plurality of short audio segments that may include manyword errors and low redundancy. Further, as opposed to news broadcasts,there may be many speakers in the teleconference each providing smallsnippets of speech that contributes to the overall spoken document.

Therefore, the same approach used for broadcast news will not providesatisfactory results if one's task is to retrieve a short snippet ofspeech in a domain where WER's can be as high as 50%. This is thesituation with teleconference speech, where one's task is to find if andwhen a participant uttered a certain phrase.

What is needed in the art are techniques that provide improved spokendocument retrieval systems for spoken documents generated from telephoneconversations or teleconferences and the like.

SUMMARY OF THE INVENTION

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. Thefeatures and advantages of the invention may be realized and obtained bymeans of the instruments and combinations particularly pointed out inthe appended claims. These and other features of the present inventionwill become more fully apparent from the following description andappended claims, or may be learned by the practice of the invention asset forth herein.

Disclosed herein is an indexing procedure for spoken utterance retrievalthat works on lattices rather than just single-best text. This procedurecan improve F scores by over five points compared to single-bestretrieval on tasks with poor WER and low redundancy. The representationis flexible so that both word lattices and phone lattices may berepresented, the latter being important for improving performance whensearching for phrases containing out of vocabulary (OOV) words.

The invention comprises systems, methods and computer-readable media forproviding a lattice-based search for spoken utterance retrieval. Aspoken document as referred to herein is preferably a document havingmoderate word error rates such as telephone calls or teleconferences.The method comprises converting speech associated with a spoken documentinto a lattice representation and indexing the lattice representation ofspeech. These steps are performed typically off-line. Upon receiving aquery from a user, the method further comprises searching the indexedlattice representation of speech and returning retrieved audio segmentsfrom the spoken document that match the user query.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and otheradvantages and features of the invention can be obtained, a moreparticular description of the invention briefly described above will berendered by reference to specific embodiments thereof which areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered to be limiting of its scope, the invention will bedescribed and explained with additional specificity and detail throughthe use of the accompanying drawings in which:

FIG. 1 illustrates a system according to an embodiment of the invention;

FIG. 2A illustrates a method embodiment of the invention;

FIG. 2B illustrates another method according to an embodiment of theinvention;

FIG. 3 shows experimental results related to precision recall using wordlattices for teleconferences;

FIG. 4 shows a comparison of word lattices and word/phone hybridstrategies for teleconferences;

FIG. 5 shows the effect of minimum pronunciation length using aword/phone hybrid strategy for teleconferencing;

FIG. 6 shows a comparison of various recognition vocabulary sizes fortelephone conversations; and

FIG. 7 shows a precision versus recall comparison for various techniqueson different tasks.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be understood by the following description ofthe various embodiments of the invention. The invention extends audiosegment retrieval techniques to scenarios such as telephone calls andteleconferencing applications. The task is locating occurrences of aquery in spoken communications to aid browsing. The approach is relatedto spoken document retrieval and word spotting. In one case, the processinvolves identifying a short segment of audio which may be termed a“document” within a larger group of audio segments. Similarly, eachaudio segment may be termed a document.

Although reasonable retrieval performance can be obtained using the bestASR hypothesis for tasks with moderate (˜20%) word error rates, taskswith higher (40-50%) word error rates require use of multiple ASRhypotheses. One aspect of the present invention is the addition of ASRlattices that make the system more robust to recognition errors. A wordlattice may be generally termed as a directed graph of words and linksbetween them which can compactly encode a large number of possiblesentences. Each word in the lattice is augmented with its observationlikelihood so that any particular path though the lattice can becombined with the prior probability derived from other language models.Timing information is also typically provided in a word lattice. See,e.g., Huang, Acero and Hon, Spoken Language Processing, Prentice HallPTR, 2001, pages 664-673. Those of skill in the art understand furtherthe details and structure of word lattices and therefore no more detailsare provided herein.

Almost all ASR systems have a closed vocabulary that relates to theparticular domain or subject matter. This restriction comes fromrun-time requirements as well as the finite amount of data used fortraining the language models of the ASR systems. Typically therecognition vocabulary is taken to be the words appearing in thelanguage model training corpus. Sometimes the vocabulary is furtherreduced to only include the most frequent words in the corpus. The wordsthat are not in this closed vocabulary—the out-of-vocabulary (OOV)words—will not be recognized by the ASR system, contributing torecognition errors. Using phonetic search helps retrieve OOV words.

FIG. 1 illustrates the basic system 10 of the apparatus embodiment ofthe invention. Speech 12 is provided to an ASR module 14. The speech 12may be generated from a telephone call, teleconference or other anyother source that has a high word error rate. Although not specificallyrequired for the present invention, it is understood that the speech 12that is provided has a higher word error rate than speech that may beobtained from a more controlled source such as broadcast news. The ASRmodule 14 converts speech into a lattice representation of the speech.The ASR module 14 may also provide timing information entries within thelattice or stored separate from the lattice. An index module 16 indexesthe lattice representation for efficient retrieval. The two steps ofconverting the speech to a lattice representation and indexing thelattice are preferably performed off-line. A search and match module 18receives the speech or other type of input 20 from a user 22 thatrepresents a query. The search and match module 18 receives the queryand searches the indexed lattice representations and locates thematching audio segments 24 and returns them to the user.

The basic system shown in FIG. 1 may be employed in many scenarios. Forexample, the modules may be programmed and operating on a singlecomputer server or on a distributed network. There is no particularprogramming language that is required to code the modules. The speechinput device 20 may be a telephone or other computing device that canreceive speech or other multi-modal input from a user 22. There may bevarious wireless and wired links between the various components of theinvention.

As an example of the benefits of the present invention, assume thespeech 12 was provided to the ASR module 14 from a teleconference of abuilder, architect and a contractor regarding the details of a house tobe built that included, among other topics, revised plans to add a hometheater room. Assume that the further that the buyer of the housedesires to receive the details of the home theater portion of theconversation. After the conference call and the processing of the callaccording to the present invention wherein the spoken documents may beindexed and retrievable, a person 22 may be able to call in via acomputing device and submit a request for the audio segment associatedwith the home theater. The query (which may be speech, text, or acombination of speech and text or other input modalities) is processedand used to identify, retrieve and return the audio portions related tothe home theater to the user 22.

Various features of the process of the present invention have preferableapproaches. For example, it is preferred that the ASR module 14 uses astate-of-the-art HMM based large vocabulary continuous speechrecognition (LVCSR) system. The acoustic models used with ASR preferablycomprise a decision tree state clustered triphones and the outputdistributions are mixtures of Gaussians. The language models arepreferably pruned backoff trigram models. The pronunciation dictionariespreferably contain few alternative pronunciations. Pronunciations thatare not in the baseline pronunciation dictionary (including OOV querywords) are preferably generated using a text-to-speech (TTS) front-end(not shown) that generates a phonetic representation of text. The TTSfront-end can produce multiple pronunciations. The ASR systems may ormay not be single pass systems. The recognition networks are preferablyrepresented as weighted finite state machines (FSMs). As can beappreciated, the above approaches present the best mode of practicingthe invention. There are alternate approaches known to those of skill inthe art that are available and within the scope of the contemplatedinvention as well.

The output of the ASR module 14 may be preferably represented as an FSMand may also be in the form of a best hypothesis string or a lattice ofalternate hypotheses. The labels on the arcs of the FSM may be words orphones, and the conversion between the two can easily be done using FSMcomposition. The costs on the arcs are negative log likelihoods.Additionally, timing information can also be present in the output.

FIG. 2A illustrates one of the method embodiments of the invention. Thisrelates to a method of retrieving a spoken document, the methodcomprises converting speech associated with a spoken document into alattice representation (202) and indexing the lattice representation(204). Upon receiving a query from a user, the method comprisessearching the indexed lattice representation of speech (206) andreturning audio segments from the spoken document that match the userquery (208).

In the case of lattices, one aspect of the invention relates to storinga set of indices, one for each arc label (word or phone) l, that recordsthe lattice number L[a], input-state k[a] of each arc a labeled with lin each lattice, along with the probability mass f(k[a]) leading to thatstate, the probability of the arc itself p(a|k[a]) and an index for thenext state. To retrieve a single label from a set of latticesrepresenting a speech corpus, one simply retrieves all arcs in eachlattice from the label index. The lattices may first be normalized byweight pushing so that the probability of the set of all paths leadingfrom the arc to the final state is 1. After weight pushing, for a givenarc a, the probability of the set of all paths containing that arc isgiven by:

${p(a)} = {{\sum\limits_{\pi \in {L:{a \in \pi}}}\; {p(\pi)}} = {{f\left( {k\lbrack a\rbrack} \right)}{p\left( a \middle| {k\lbrack a\rbrack} \right)}}}$

namely, the probability of all paths leading into that arc, multipliedby the probability of the arc itself. For a lattice L a “count” C(l|L)is constructed for a given label l using the information stored in theindex I(l) as follows,

$\begin{matrix}{{C\left( l \middle| L \right)} = {\sum\limits_{\pi \in L}\; {{p(\pi)}{C\left( l \middle| \pi \right)}}}} \\{= {\sum\limits_{\pi \in L}\; \left( {{p(\pi)}{\sum\limits_{a \in \pi}\; {\delta \left( {a,l} \right)}}} \right)}} \\{= {\sum\limits_{a \in L}\; \left( {{\delta \left( {a,l} \right)}{\sum\limits_{\pi \in {L:{a \in \pi}}}\; {p(\pi)}}} \right)}} \\{= {\sum\limits_{{{a \in {I{(l)}}}:{L{\lbrack a\rbrack}}} = L}\; {p(a)}}} \\{= {\sum\limits_{{{{a \in {I{(l)}}}:{{L9}\; a}})} = L}\; {f\left( {{k\lbrack a\rbrack}{p\left( a \middle| {k\lbrack a\rbrack} \right)}} \right.}}}\end{matrix}$

where C(lπ) is the number of times l is seen on path and π and δ(a, l),is 1 if arc a has the label l and 0 otherwise. Retrieval can bethresholded so that matches below a certain count are not returned.

To search a multi-label expression (e.g. a multi-word phrase) w₁ w₂ . .. w_(n), the system seeks on each label in the expression, and then foreach (w_(i), w_(i+1)) join the output states of w_(i) with the matchinginput states of w_(i+1); in this way the system retrieves just thosepath segments in each lattice that match the entire multi-labelexpression. The probability of each match is defined asf(k[a₁])p(a₁|k[a₁]p(a₂|k[a₂]) . . . p)a_(n)|k[a_(n)]), wherep(a_(i)|k[a_(i)]) is the probability of the ith arc in the expressionstarting in arc a₁. The total “count”' for the lattice is computed asdefined above.

Note that in the limited case where each lattice is an unweighted singlepath—i.e. a string of labels—the above scheme reduces to a standardinverted index. In order to deal with queries that contain OOV words thepresent invention uses sub-word units for indexing. One sub-wordcomponent may be phones. There are two methods for obtaining phoneticrepresentation of an input utterance.

First, phone recognition using an ASR system where recognition units arephones. This is achieved by using a phone level language model insteadof the word level language model used in the baseline ASR system.Second, another aspect is converting the word level representation ofthe utterance into a phone level representation. This is achieved byusing the baseline ASR system and replacing each word in the output byits pronunciation(s) in terms of phones.

Phone recognition may be less accurate than word recognition. On theother hand, the second method can only generate phone strings that aresubstrings of the pronunciations of in-vocabulary word strings. Analternative to improving on the limitations of each of these approachesis to use hybrid language models used for OOV word detection.

For retrieval, each query word is converted into phone string(s) byusing its pronunciation(s). The phone index can then be searched foreach phone string. Note that this approach will generate many falsealarms, particularly for short query words, which are likely to besubstrings of longer words. In order to for control for this, a bound onminimum pronunciation length can be utilized. Since most short words arein vocabulary this bound has little effect on recall.

Another aspect of the invention is shown in FIG. 2B for the scenariowhere a word index and a sub-word index are available. This aspectutilizes both of the indexes to improve on the process. Upon receiving auser query (220), the method comprises searching both the word index(222) and the sub-word index (224) and combining the results to retrievethe audio segments from the spoken document that match the user's query(226).

Alternately, upon receiving a user query (220), the method may comprisesearching the word index for in-vocabulary queries (228) and searchingthe sub-word index for OOV queries (230). Yet another alternative isupon receiving a user query (220), the method comprises searching theword index and if no result is returned search the sub-word index (232).

In the first case, if the indices are obtained from ASR best hypotheses,then the result combination is a simple union of the separate sets ofresults. However, if indices are obtained from lattices, then inaddition to taking a union of results, retrieval can be done using acombined score. Given a query q, let C_(w)(q) and C_(p)(q) be thelattice counts obtained from the word index and the phone indexrespectively. The normalized lattice count is defined for the phoneindex as

${C_{p}^{norm}(q)} = \left( {{Cp}(q)} \right)^{\frac{1}{{{pron}{(q)}}}}$

where |pron(q)| is the length of the pronunciation of query q. Thecombined score is then defined to be

C _(wp)(q)=C _(w)(q)+λC ^(norm) _(p)(q)

where λ is an empirically determined scaling factor. In the other cases,instead of using two different thresholds, a single threshold on Cw(q)and C^(norm) _(p)(q) may be used during retrieval.

For evaluating ASR performance, the standard word error rate (WER) maybe used as a metric. Since retrieval is the goal, the OOV rate is usedby type to measure the OOV word characteristics. For evaluatingretrieval performance, precision and recall with respect to manualtranscriptions are used. Let Correct (q) be the number of times thequery q is found correctly, Answer (q) be the number of answers to thequery q, and Reference (q) be the number of times q is found in thereference.

${{Precision}\mspace{14mu} (q)} = \frac{{Correct}\mspace{14mu} (q)}{{Answer}\mspace{14mu} (q)}$${{Recall}\mspace{14mu} (q)} = \frac{{Correct}\mspace{14mu} (q)}{{Reference}\mspace{14mu} (q)}$

The system computes precision and recall rates for each query and reportthe average over all queries. The set of queries Q consists of all thewords seen in the reference except for a stoplist of 100 most commonwords.

${Precision} = {\frac{1}{Q}{\sum\limits_{q \in Q}\; {{Precision}\mspace{14mu} (q)}}}$${Recall} = {\frac{1}{Q}{\sum\limits_{q \in Q}\; {{Recall}\mspace{14mu} (q)}}}$

For lattice based retrieval methods, different operating points can beobtained by changing the threshold. The precision and recall at theseoperating points can be plotted as a curve. In addition to individualprecision-recall values, the system also computes the F-measure definedas

$F = \frac{2 \times {Precision} \times {Recall}}{{Precision} + {Recall}}$

and reports the maximum F-measure (maxF) to summarize the information ina precision-recall curve.

Three different corpora are used to assess the effectiveness ofdifferent retrieval techniques. The first corpus is the DARPA BroadcastNews corpus consisting of excerpts from TV or radio programs includingvarious acoustic conditions. The test set is the 1998 Hub-4 BroadcastNews (hub4e98) evaluation test set (available from LDC, Catalog no.LDC2000S86) which is 3 hours long and was manually segmented into 940segments. It contains 32411 word tokens and 4885 word types. For ASR, areal-time system may be used. Since the system was designed for SDR, therecognition vocabulary of the system has over 200,000 words.

The second corpus is the Switchboard corpus consisting of two partytelephone conversations. The test set is the RT02 evaluation test setwhich is 5 hours long, has 120 conversation sides and was manuallysegmented into 6266 segments. It contains 65255 word tokens and 3788word types. For ASR, the first pass of the evaluation system was used.The recognition vocabulary of the system has over 45,000 words.

The third corpus is named Teleconferences since it consists ofmultiparty teleconferences on various topics. The audio from the legs ofthe conference are summed and recorded as a single channel. A test setof six teleconferences (about 3.5 hours) was transcribed. It contains31106 word tokens and 2779 word types. Calls are automatically segmentedinto a total of 1157 segments prior to ASR, using an algorithm thatdetects changes in the acoustics. The first pass of the Switchboardevaluation system was used for ASR.

Table 1 shows the ASR performance on these three tasks as well as theOOV Rate by type of the corpora. This table illustrates the word errorrate (WER) and OOV Rate by type of various LVCSR tasks. It is importantto note that the recognition vocabulary for the Switchboard andTeleconferences tasks are the same and no data from the Teleconferencestask was used while building the ASR systems.

TABLE 1 Task WER OOV Rate by Type Broadcast News ~20% 0.6% Switchboard~40%   6% Teleconferences ~50%  12%

As a baseline, the best word hypotheses of the ASR system are used forindexing and retrieval. The performance of this baseline system is givenin Table 1. As expected, very good performance is obtained on theBroadcast News corpus. It is interesting to note that when moving fromSwitchboard to Teleconferences the degradation in precision-recall isthe same as the degradation in WER.

TABLE 2 Task WER Precision Recall Broadcast News ~20% 92% 77%Switchboard ~40% 74% 47% Teleconferences ~50% 65% 37%

The second set of experiments investigated the use of ASR word lattices.In order to reduce storage requirements, lattices can be pruned tocontain only the paths whose costs (i.e. negative log likelihood) arewithin a threshold with respect to the best path. The smaller this costthreshold is, the smaller the lattices and the index files are. FIG. 3illustrates the precision-recall curves 302 for different pruningthresholds on the Teleconferences task.

Table 3 shows the resulting index sizes and maximum F-measure values. Onthe teleconferences task, it was observed that cost=6 yields goodresults, and used this value for the rest of the experiments.

Note that this increases the index size with respect to the ASR 1-bestcase by 3 times for Broadcast News, by 5 times for Switchboard and by 9times for Teleconferences.

TABLE 3 Task Pruning Size maxF Broadcast News nbest = 1 29 84.0Broadcast News cost = 6 91 84.8 Switchboard nbest = 1 18 57.1Switchboard cost = 6 90 58.4 Teleconferences nbest = 1 16 47.4Teleconferences cost = 2 29 49.5 Teleconferences cost = 4 62 50.0Teleconferences cost = 6 142 50.3 Teleconferences cost = 12 3100 50.1

Next, investigations compared using the two methods of phonetictranscription discussed above—phone recognition and word-to-phoneconversion—for retrieval using only phone lattices. In Table 4 theprecision and recall values that yield the maximum F-measure as well asthe maximum F-measure values are presented. These results clearlyindicate that phone recognition is inferior to other approaches.

TABLE 4 Source for Indexing Precision Recal maxF Phone Recognition 25.637.3 30.4 Conversion from Words 43.1 48.5 45.6

The strategy of searching the word index, if no result is returnedsearch the phone index, is preferred to the other strategies. Table 5compares the maximum F-values for the three strategies for using wordand phone indices.

TABLE 5 Strategy maxF 1. combination 50.5 2. vocabulary cascade 51.0 3.search cascade 52.8

FIG. 4 presents results 402 for this strategy on the Teleconferencescorpus. The phone indices used in these experiments were obtained byconverting the word lattices into phone lattices. Using the phoneindices obtained by phone recognition gave significantly worse results.

When searching for words with short pronunciations in the phone indexthe system will produce many false alarms. One way of reducing thenumber of false alarms is to disallow queries with short pronunciations.FIG. 5 show the effect of imposing a minimum pronunciation length forqueries 502. For a query to be answered its pronunciation has to havemore than minphone phones, otherwise no answers are returned. Bestmaximum F-measure result is obtained using minphone=3. Thus, this figureshows the effect of minimum pronunciation length using a word/phonehybrid strategy for teleconferences.

FIG. 6 presents results 602 for different recognition vocabulary sizes(5k, 20k, 45k) on the Switchboard corpus. The OOV rates by type are 32%,10% and 6% respectively. The word error rates are 41.5%, 40.1% and 40.1%respectively. The precision recall curves are almost the same for 20,000and 45,000 vocabulary sizes.

So far, in all the experiments the query list consisted of single words.In order to observe the behavior of various methods when faced withlonger queries, a set of word pair queries was used in a study. Insteadof using all the word pairs seen in the reference transcriptions, theones which were more likely to occur together were chosen than withother words. For this, the word pairs (w₁,w₂) were sorted according totheir pointwise mutual information

$\log \frac{p\left( {w_{1},w_{2}} \right)}{{p\left( w_{1} \right)}{p\left( w_{2} \right)}}$

and used the top pairs as queries in our experiments.

As it turns out, the precision of the system is very high on this typeof queries. For this reason, it is more interesting to look at theoperating point that achieves the maximum F-measure for each technique,which in this case coincides with the point that yields the highestrecall. Table 6 presents results on the Switchboard corpus using 1004word pair queries. Using word lattices it is possible to increase therecall of the system by 16.4% while degrading the precision by only2.2%. Using phone lattices another 3.7% increase can be achieved inrecall for 1.2% loss in precision. The final system still has 95%precision.

TABLE 6 System Precision Recall F- Word 1-best 98.3 29.7 45.6 Wordlattices 96.1 46.1 62.3 Word + Phone lattices 94.9 49.8 65.4

Finally, a comparison of various techniques on different tasks is shownin Table 7 where the maximum F-measure (maxF) is given. Using wordlattices yields a relative gain of 3-5% in maxF over using best wordhypotheses. For the final system that uses both word and phone lattices,the relative gain over the baseline increases to 8-12%.

FIG. 7 presents the precision recall curves 702. The gain from usingbetter techniques utilizing word and phone lattices increases asretrieval performance gets worse. FIG. 7 shows the precision recall forvarious techniques on different tasks. The tasks are Broadcast News (+),Switchboard (x), and Teleconferences (o). The techniques are using bestword hypotheses (single points), using word lattices (solid lines), andusing word and phone lattices (dashed lines).

TABLE 7 System Task 1-best W Lats W + P Lats Broadcast News 84.0 84.886.0 Switchboard 57.1 58.4 60.5 Teleconferences 47.4 50.3 52.8

Disclosed herein is an indexing procedure for spoken utterance retrievalthat works on ASR lattices rather than just single-best text. It wasdemonstrated that this procedure can improve maximum F-measure by overfive points compared to single-best retrieval on tasks with poor WER andlow redundancy. The representation is flexible so that both wordlattices, as well as phone lattices, can be represented, the latterbeing important for improving performance when searching for phrasescontaining OOV words. It is important to note that spoken utteranceretrieval for conversational speech has different properties than spokendocument retrieval for broadcast news. Although consistent improvementswere observed on a variety of tasks including Broadcast News, theprocedure proposed here is most beneficial for more difficultconversational speech tasks like Switchboard and Teleconferences.

Embodiments within the scope of the present invention may also includecomputer-readable media for carrying or having computer-executableinstructions or data structures stored thereon. Such computer-readablemedia can be any available media that can be accessed by a generalpurpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium which can be used to carryor store desired program code means in the form of computer-executableinstructions or data structures. When information is transferred orprovided over a network or another communications connection (eitherhardwired, wireless, or combination thereof) to a computer, the computerproperly views the connection as a computer-readable medium. Thus, anysuch connection is properly termed a computer-readable medium.Combinations of the above should also be included within the scope ofthe computer-readable media.

Computer-executable instructions include, for example, instructions anddata which cause a general purpose computer, special purpose computer,or special purpose processing device to perform a certain function orgroup of functions. Computer-executable instructions also includeprogram modules that are executed by computers in stand-alone or networkenvironments. Generally, program modules include routines, programs,objects, components, and data structures, etc. that perform particulartasks or implement particular abstract data types. Computer-executableinstructions, associated data structures, and program modules representexamples of the program code means for executing steps of the methodsdisclosed herein. The particular sequence of such executableinstructions or associated data structures represents examples ofcorresponding acts for implementing the functions described in suchsteps.

Those of skill in the art will appreciate that other embodiments of theinvention may be practiced in network computing environments with manytypes of computer system configurations, including personal computers,hand-held devices, multi-processor systems, microprocessor-based orprogrammable consumer electronics, network PCs, minicomputers, mainframecomputers, and the like. Embodiments may also be practiced indistributed computing environments where tasks are performed by localand remote processing devices that are linked (either by hardwiredlinks, wireless links, or by a combination thereof) through acommunications network. In a distributed computing environment, programmodules may be located in both local and remote memory storage devices.

Although the above description may contain specific details, they shouldnot be construed as limiting the claims in any way. Other configurationsof the described embodiments of the invention are part of the scope ofthis invention. Accordingly, the appended claims and their legalequivalents should only define the invention, rather than any specificexamples given.

We claim:
 1. A method comprising: indexing a lattice representation ofspeech in a spoken document, to yield an indexed lattice representation;searching, via a processor, the indexed lattice representation based ona query associated with the spoken document, to yield one or more searchresults; and based on the one or more search results, returning one ormore audio segments from the spoken document which correspond to thequery.
 2. The method of claim 1, further comprising: receiving the queryfrom a user via a device.
 3. The method of claim 1, further comprising:searching a word index of the spoken document using the query, to yieldthe one or more search results.
 4. The method of claim 1, furthercomprising: searching a sub-word index of the spoken document using thequery, to yield the one or more search results.
 5. The method of claim3, wherein searching the word index of the spoken document using thequery further comprises searching the word index when the query isin-vocabulary.
 6. The method of claim 4, wherein searching the sub-wordindex of the spoken document using the query further comprises searchingthe sub-word index when the query is out-of-vocabulary.
 7. The method ofclaim 1, wherein the query comprises at least one query word.
 8. Asystem comprising: a processor; and a computer-readable storage mediumhaving instructions stored which, when executed by the processor, causethe processor to perform operations comprising: indexing a latticerepresentation of speech in a spoken document, to yield an indexedlattice representation; searching the indexed lattice representationbased on a query associated with the spoken document, to yield one ormore search results; and based on the one or more search results,returning one or more audio segments from the spoken document whichcorrespond to the query.
 9. The system of claim 8, wherein thecomputer-readable storage medium stores additional instructions storedwhich, when executed by the processor, cause the processor to performoperations further comprising: receiving the query from a user via adevice.
 10. The system of claim 8, wherein the computer-readable storagemedium stores additional instructions stored which, when executed by theprocessor, cause the processor to perform operations further comprising:searching a word index of the spoken document using the query, to yieldthe one or more search results.
 11. The system of claim 8, wherein thecomputer-readable storage medium stores additional instructions storedwhich, when executed by the processor, cause the processor to performoperations further comprising: searching a sub-word index of the spokendocument using the query, to yield the one or more search results. 12.The system of claim 10, wherein searching the word index of the spokendocument using the query further comprises searching the word index whenthe query is in-vocabulary.
 13. The system of claim 11, whereinsearching the sub-word index of the spoken document using the queryfurther comprises searching the sub-word index when the query isout-of-vocabulary.
 14. The system of claim 8, wherein the querycomprises at least one query word.
 15. A computer-readable storagedevice having instructions stored which, when executed by a processor,cause the processor to perform operations comprising: indexing a latticerepresentation of speech in a spoken document, to yield an indexedlattice representation; searching the indexed lattice representationbased on a query associated with the spoken document, to yield one ormore search results; and based on the one or more search results,returning one or more audio segments from the spoken document whichcorrespond to the query.
 16. The computer-readable storage device ofclaim 15, wherein the computer-readable storage device stores additionalinstructions stored which, when executed by the processor, cause theprocessor to perform operations further comprising: receiving the queryfrom a user via a device.
 17. The computer-readable storage device ofclaim 15, wherein the computer-readable storage device stores additionalinstructions stored which, when executed by the processor, cause theprocessor to perform operations further comprising: searching a wordindex of the spoken document using the query, to yield the one or moresearch results.
 18. The computer-readable storage device of claim 15,wherein the computer-readable storage device stores additionalinstructions stored which, when executed by the processor, cause theprocessor to perform operations further comprising: searching a sub-wordindex of the spoken document using the query, to yield the one or moresearch results.
 19. The computer-readable storage device of claim 17,wherein searching the word index of the spoken document using the queryfurther comprises searching the word index when the query isin-vocabulary.
 20. The computer-readable storage device of claim 18,wherein searching the sub-word index of the spoken document using thequery further comprises searching the sub-word index when the query isout-of-vocabulary.